Recognizing an uninvited egg

Some cuckoo species lay their eggs in the nests of other birds, leaving the hosts to feed the chicks. This reproductive strategy, called brood parasitism, can be costly for a host. A cuckoo chick may remove the host’s eggs or young, so the parents spend their energy raising an unrelated bird.

Hosts of some species reject eggs that differ from their own in color or markings. This ability puts cuckoos with poorly matching eggs at a disadvantage: their offspring are less likely to survive. Over many generations, eggs that resemble those of the host can become more common among the cuckoos.

A close match, however, does not necessarily settle the contest. In some host species, females produce eggs with distinctive combinations of markings. Eggs laid by one female resemble each other but differ from those of her neighbors. A cuckoo egg that resembles the average egg of the host species may therefore stand out in a particular nest.

These individual patterns function like signatures, making an imitation easier to detect. The interaction illustrates coevolution: evolutionary change in one species creates pressure for change in another. Neither bird consciously designs the next improvement; differences in reproductive success shape the characteristics of later generations.